076
FXUS63 KLSX 111737
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1237 PM CDT Mon May 11 2026

.KEY MESSAGES...

- Moderating temperatures are expected through Tuesday ahead of a cold
  front. There is a chance (30-70%) of showers and thunderstorms
  Tuesday afternoon/evening with the frontal passage.

- After a brief cool down Wednesday - Thursday, above-normal
  temperatures are forecast beginning Friday.


&&

.SHORT TERM...  (Through Late Tonight)
Issued at 236 AM CDT Mon May 11 2026

A surface ridge of high pressure will remain draped across the
bi- state area today, helping provide a dry day with plenty of
sunshine. High temperatures are expected to top out mainly in the
mid 70s, or right about average for the middle of May.

The aforementioned surface ridge is forecast to push off into the
Ohio Valley late tonight, with southerly return flow beginning in
its wake. This should help lead to a milder night, with lows ranging
from the upper 40s to mid 50s. The coolest conditions are expected
to be across portions of southeast Missouri and southwest Illinois
where winds will stay light/variable the longest.


Gosselin

&&

.LONG TERM...  (Tuesday through Sunday)
Issued at 236 AM CDT Mon May 11 2026

(Tuesday - Tuesday Night)

A quick warmup will occur on Tuesday as strong low-level warm air
advection continues ahead of an approaching cold front. High
temperatures mostly in the low 80s are forecast, or about 5-10
degrees above normal.

Convective initiation will be the focus late Tuesday
afternoon/evening along the cold front. Thunderstorms tend to
initiate where surface convergence and mid/upper level forcing for
ascent is sufficiently strong, and this should be no exception.
While the strongest surface convergence and mid/upper level forcing
for ascent will be across the Great Lakes, showers and thunderstorms
are more likely than not to develop as far southwest as near/east of
the Mississippi River. Deep-layer shear will be quite strong (40-45
knots), but there is uncertainty in moisture return/instability.
There really isn`t a deep Gulf connection by any means, with more
"recycled" Gulf moisture trying to advect northeastward and pool
along the boundary. Relatively modest dewpoints (mid to upper 50s)
are most likely along the front. Midlevel lapse rates are also not
expected to be too steep, which also should help limit instability.
Probabilities for at least 500 J/kg of SBCAPE on the 0Z HREF peak
out in the 30-60% range across parts of central/northeast Missouri,
but fall to 10-20% for at least 1000 J/kg. The (relatively) higher
instability is also forecast to be further to the south and west,
where convective initiation is more doubtful as well. Therefore,
there still does not seem to be a clear signal for any organized
strong to severe thunderstorms in our area. Small hail and perhaps
some gusty winds (if the storms are surface based) are possible, but
severe hail likely would necessitate supercells to develop. A
supercell or two cannot be ruled out, but this does not seem to be a
likely scenario. While the effective shear should be more than high
enough to support supercells, instability probably will be too low.
The supercell composite parameter (SCP) forecast values, even on the
bullish GFS with respect to instability, largely remain near or
below 1.


(Wednesday - Thursday)

A brief cooldown is expected behind the cold front through Thursday.
The air mass has trended back slightly cooler compared to 24 hours
ago. The incoming air mass is not too noteworthy, but does seem
supportive of near to slightly below normal temperatures. The
incoming surface high on the NAEFS is near 1020 hPa, or ~90th
percentile of climatology. Temperature anomalies at 850 hPa on both
the GEFS and EPS also are in the -3 to -6C range, with surface
dewpoints (40s) in the 10th-25th percentile range. High temperatures
each afternoon are forecast to be mostly in the 70s, with an east
(cooler) to west (warmer) gradient each day. Lows Wednesday
night/Thursday morning are expected to be in the 40s, or about 5-8F
below normal.


(Thursday Night - Sunday)

Another warmup will commence Thursday night as low-level warm air
advection intensifies on the western periphery of the departing
surface ridge. There is very high confidence in well-above normal
low temperatures through the end of the forecast period, but there
continues to be a lot of uncertainty with specifics on high
temperatures each day. The mid/upper level pattern late week into
this weekend is very likely to be quasi-zonal, with multiple
weak/subtle midlevel shortwave troughs potentially moving across the
Plains through the mid-Mississippi Valley. High temperatures are
most likely going to be in the 80s, though a well-timed shortwave
could yield enough cloud cover and rain chances to keep highs in the
mid to upper 70s for portions of the area. The chances of 90+ degree
highs remains low at this time. The air mass moving into the area
looks warm, but not overly impressive. Temperatures at 850 hPa on
the LREF climb closer to +15C, but that is ~90th percentile of
climatology. The mid/upper level ridging also is expected to stay to
our south, with 500-hPa heights staying well below the 90th
percentile. Probabilities for 90+ degree highs each day on the LREF
remain at or below 15% and look much more representative compared to
the NBM.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1236 PM CDT Mon May 11 2026

Dry and VFR flight conditions are expected to persist through
Tuesday afternoon at all terminals. Patchy river valley fog cannot
entirely be ruled out at KSUS tonight, but confidence is low in its
occurrence. Otherwise, winds will become southwesterly and
strengthen Tuesday morning as a surface high pressure center departs
and a cold front approaches from the northwest.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX